CHICO, Calif. — Some learned about the looming wildfire from neighbors knocking on their doors. Or frantic cellphone calls from friends. Others just looked out their windows and saw the smoke and flames, or heard the chaos of neighbors hustling up children and pets and scrambling to get out.

Matthew White was sound asleep when the fire began raging around his home in Paradise, Calif., the morning of Nov. 8. But somehow he heard his cellphone ring.

It was a friend of his shouting on the other end of the line: “Get the hell up and get the hell out! Paradise is on fire!”

In the frenzied first hours of the Camp Fire, which reduced Paradise to ashes and is the deadliest wildfire in modern California history, only a fraction of people living near the fire received alerts or evacuation orders from local authorities. Paradise was already seen as highly vulnerable to fire, with few roads in and out, and with many homes built in forests, not subdivisions.

In the weeks and months ahead, officials across the state will grapple with the question of whether more people could have been alerted sooner, perhaps saving more lives.

Many of those who barely made it to safety already have their answer.

“They totally dropped the ball on this,” Mr. White said, of the authorities. “Look, all these people dead, all these people missing. It’s like they decided to forget about us. Like we weren’t worth saving.”

The decision to issue alerts and evacuation orders rests with local authorities, and as the Camp Fire began on Nov. 8, the Butte County Sheriff’s Department decided to use what experts say is an outdated system — called Code Red — to notify residents of danger with a phone call.

But only residents who sign up for the service receive alerts — and only a fraction of them had. The decision not to issue an Amber Alert-style message, a federal government system that could reach all cellular phones in the area, was partly out of fear of causing panic and traffic jams on the one main roadway out of Paradise, according to Kory L. Honea, the Butte County sheriff.

For Canvis Villaneuva, it was the children next door running around in a panic that alerted her to danger. “And then my neighbor started yelling at me that we need to evacuate,” she said. “I thought, ‘evacuate what?’ Then I saw the flames.”

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Paradise, a hilly and wooded community in the Sierra Nevada, is connected to the outside world by the main road, called Skyway, and a few other smaller roads. Even without the broader warnings, panicked residents spent hours stuck on Skyway trying to escape to Chico, the city in the valley below, and some died as flames engulfed their cars.

“I understand there are people who say they didn’t get enough notice — I agree,” Sheriff Honea said in an interview this week. “There was not enough notice. We couldn’t have given enough notice given the circumstances.”

Similar questions were raised during the wine country fires in Sonoma County in Northern California last year. The Tubbs Fire was briefly the state’s most destructive wildfire, with more than 5,500 buildings burned. Officials there also decided against sending out a mass alert to every cellphone in the region because they feared it would clog up roads both for evacuees and emergency vehicles. And in the aftermath of deadly mudslides earlier this year in Montecito, which killed 21 people, the authorities were sharply criticized for not issuing mandatory evacuation notices.

In last year’s Tubbs Fire, warnings were sent out to those who subscribed to a police notification system known as Nixle.

“We know from the Tubbs Fire of a year ago that subscription rates are low,” said Thomas Cova, a geography professor at the University of Utah who specializes in emergency management. “They discovered in hindsight that they were reaching very few homes.”

This was a lesson, he said, that was not applied as disaster unfolded in Paradise.

“It’s really disappointing,” he said. “You’d think with what happened in Sonoma County, that other counties in California and elsewhere would review their systems and go door-to-door to get everyone subscribed, or whatever it takes.”

Professor Cova said that while the authorities in California were slow to grasp the lessons learned from the Tubbs Fire or Montecito, the scope of the loss from the Camp Fire is likely to lead to drastic changes in how communities respond to disasters.

“I think we have a watershed moment, in terms of loss of life,” he said. “Not since Katrina have we seen this much loss of life from a disaster.”

Professor Cova said it is a myth — not backed by any research — that notifying the public immediately at times of disaster, whether it be a wildfire or a hurricane, backfires by causing panic and hampering emergency efforts.

“It’s not lack of technology or technological know-how,” he said, of the repeated failure of the authorities in California to issue immediate mass alerts at the first sign of danger. “There are just these beliefs that warning everyone too quickly is a bad thing.”

Sheriff Honea argued that law enforcement officials are condemned no matter how they handle evacuations.

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In February 2017, he was criticized for ordering an evacuation in a different crisis. With fears that the spillway of the Oroville Dam, the highest in California, might give way, around 180,000 people living below the dam were ordered to leave their homes. The mass evacuation caused a panicked rush onto roads and highways.

“One of the criticisms or the lessons learned was, hey, try to stagger your evacuations,” Sheriff Honea said. In Paradise, the sheriff ordered staged evacuations according to zones in immediate peril. If everyone had been ordered to leave at once, the evacuation may have been more perilous, he said.

“If we hadn’t tried to evacuate those zones with an imminent threat versus the ones where the threat was progressing towards them, I believe the traffic would have been even worse,” he said.

Paradise, given how it was planned and developed over decades, with homes built on a ridge surrounded by forests and just a handful of roads out, seemed poised to one day face disaster. The town studied evacuation routes after a 2008 wildfire in the area. The Paradise mayor, Jody Jones, is a traffic specialist who spent years working for the state agency that manages roads. But the mayor and other law enforcement officials argued that the Camp Fire spread so quickly — at one point moving the equivalent length of a football field per second — that it overwhelmed the ability of emergency workers.

After the 2008 wildfire, a Butte County grand jury report noted the difficulty residents had in evacuating, and advised that the authorities find ways to make it easier for residents to escape. Instead, the authorities narrowed Skyway in the center of town to improve pedestrian safety, reduce traffic and promote commerce — a decision that is sure to be scrutinized in the aftermath of the Camp Fire. Eric Reinbold, the police chief of Paradise, was skeptical that removing a lane from parts of the downtown area made the evacuation more difficult. “I don’t know that it would have made a difference or not,” Chief Reinbold said. “You just can’t evacuate that many people in minutes or hours.”

A recent editorial in the The Chico Enterprise-Record described the town’s preoccupation with emergency evacuations in recent years. “People prepared. Fire prevention officials planned. They drilled. They worked with homeowners. They invented fire-safe councils and Fire on the Ridge and sent fire prevention officials to schools via a program called Fire Pals. They raised money to keep fire lookouts open when the state said it wouldn’t.

“Eventually, geography and topography proved to be the trap everyone thought it was.”

John Owens counts himself among the lucky residents of Paradise. He and his wife and their dog are now sleeping in a friend’s car in the parking lot of a Burger King in Chico. Their home in a trailer park in Paradise was destroyed, as was his Ford pickup, for which he had no insurance.

“But we’re alive, man,” said Mr. Owens, 57, a welder. He said he had been working in Chico on the day of the fire when he got a call from his wife, who was at home.

Mr. Owens said neither of them received alerts about the fire. Sitting on a park bench in Chico, he wondered if the authorities could have done a better job of warning people.

“How can anyone prepare for something this enormous?” he asked. “There are just some things you maybe can’t get a handle on ahead of time. This could just be God’s way of staying away from a place that’s bound to burn.”